1. Field of the Invention
The present invention relates to an acoustic sensor and method of detecting acoustic signals, and more particularly, to both a Sagnac interferometric sensor array for acoustic sensing and method of detecting acoustic signals in which simultaneous measurement of acoustic signals originating from multiple sources is possible.
2. Description of the Prior Art
Fiber-optic interferometric sensors are based upon the phase modulation in light passing through two fiber optic paths which occurs when those fibers are exposed to physical quantities to be measured. The light beams/pulses having passed through the fiber optic paths interfere with each other, forming an interference signal which has intensity variations due to the phase difference between the light beams/pulses. The information about the physical quantities to be measured can be obtained from the signal processing of the intensity variations. Such fiber-optic interferometric sensors have the superior sensitivity to that of a conventional generic sensor and can be used in the chemically hazardous environments.
A conventional fiber-optic interferometric array for acoustic sensing has been constructed only in a configuration of Mach-Zehnder or Michelson interferometer in order to obtain flat response (phase difference caused by a unit acoustic pressure per a unit length of optical fiber) at acoustic frequencies. It exhibits a relatively good response at low acoustic frequencies. However, it is subjected to influence of the intensity noise created by the phase fluctuation of light source, which is attributed mainly to mismatch of optical path length, and shows a significant phase drift arising from environmental perturbation such as temperature. Thus, it is inevitable to employ a complex scheme of signal processing for data acquisition and further it might loose the signal completely due to random polarization change of interfering lights.
On the other hand, in a conventional acoustic sensor composed with Sagnac interferometer there is no source phase-induced intensity noise because of its intrinsic complete path balance and the signal fading due to variation of polarization is avoided easily. However, its responsivity is dependent upon the frequency of acoustic signal and decreases at low frequencies. So, only a single Sagnac interferometer-type sensor has been studied in the academic point of view.
The architecture of a single Sagnac interferometric acoustic sensor is presented and a way of signal processing is explained as follows.
FIG. 1 is a schematic diagram for a single Sagnac interferometer of acoustic sensor associated with an ideal 3.times.3 directional coupler at the input and output ports for signal processing. Referring to FIG. 1, light from a light source 100 is divided by a 3.times.3 directional coupler 120 into two light beams and propagates around a Sagnac loop 130. One of the light beams in a Sagnac loop 130 circles clockwise and the other does counterclockwise. The light beams propagating in the opposite directions from each other recombine at the 3.times.3 directional coupler 120 and produce interference light. Then, the output signals of interference are detected by a first and a second photodetectors 160 and 170, and are input to a differential amplifier 180 to produce a difference between interference output intensities.
In the above configuration of acoustic sensor, if an acoustic signal is applied to a fiber-optic sensing coil 140, a phase difference is generated in two counter-propagating light beams. In addition to the phase difference .DELTA..phi.(t) caused by an applied acoustic signal, an extra phase bias of .+-.2/3.pi. generated in the 3.times.3 directional coupler 120 is augmented. The output intensities I.sub.1 and I.sub.2 detected at the first and the second detectors 160 and 170 are represented as following equations 1 and 2. ##EQU1##
where P.sub.0 is the input light intensity, .DELTA..phi.(t) is the phase difference caused by an applied acoustic signal, and R is the responsivity of the photodetector.
When the polarization states of two light beams are identical, the difference I.sub.diff of interference output intensities after the differential amplifier is expressed as following equation 3. ##EQU2##
equation 3 can be approximated as in Equation 4 for a small value of .DELTA..phi.(t). ##EQU3##
Since the difference in interference output intensities I.sub.diff is proportional to .DELTA..phi.(t), the applied acoustic signal can be gained by measuring I.sub.diff.
However, in order to construct such a device, it is inevitable to use an ideal 3.times.3 directional coupler and two photodetectors of high accuracy. In addition, if this device is used to construct an array of sensors, the circuitry for signal processing becomes complex because the same number of differential amplifiers as sensors are necessary to use. It is also very difficult to make two interference outputs arrive at the differential amplifier with the same transit time.
Despite difficulties described above, if delay lines are incorporated in the Sagnac interferometer, the responsivity similar to that of Mach-Zehnder interferometer can be obtained in frequencies except at extremely low frequencies. For some frequency range, it is possible to achieve better responsivity.